Automatic gain control system



May 6, 194l- A. R. APPLEGARTH, -JR 2,240,601

AUTOMATIC GAIN CONTROL SYSTEM Filed May 24, 1939 2 Sheets-Sheet l 10050NPL /F/EP IL ZHIT.

May @1941- l A. R. APPLEGARTH, JR 2,240,601

AUTOMATIC GAIN CONTROL SYSTEM 2 Sheets-.Sheet 2 Filed May 24, 1939Patented May 6, 194i UNITED STATES PATENT FFCE i A 2,240,601- i iAUTOMATIC GAiN CONTROL SYSTEEM Alexander R. Applegarth, Jr.,Philadelphia, Pa.,

a corporation of Delaware Application May 2.4, 1939, seran No. 275,525A5 claims. (ol. 17a-7.3)

y This invention relates to a method of andV apparatus for automaticallycontrollingthe gain of an electrical signaling system. This applicationis a continuation in part of copending application Serial N0. 219,643,led Julygl, 1938. ,a

For reasons which will appear presently, the present invention isparticularly adapted for use in a television receiver, though it is byno means restricted to this use and presents advantages Which aredesirable in automatic gain control systems in general.

One object of the invention is to provide an improved system forobtaining delayed automatic gain control in an electrical signalingsystem.

Another object of the invention is to provide anl automaticgain controlsystem in which an exceptionally large change in control voltage isobtained for a given change in signal amplitude, whereby a very flatcontrol characteristic results.

A further object of the invention is to provide a novel automatic gaincontrol system which functions to maintain constant the peak valut,`rather than the average value of a modulated carrier signal. y

, A still further object is to provide, in a television system of thetype employing a signal comprising video and synchronizing componentsestablished on opposite sides of a predetermined blanking level, anautomatic gain control system which tends to maintain the blanking leveland the amplitudes of the synchronizing pulses constant, and whichyields an unusually large variation in control voltage for a givenchange in signal strength. I y

In order to understand fully the operation of a system in accordancewith the invention, reference may be had to the following descriptionand the accompanying drawings in which:

Fig. l shows a specific embodiment of the invention applied to atelevision receiver; and

Figs. 2 and 3 are diagrams which explain the operation of the systemaccording to the invention.

In Fig. 1, the incoming modulated carrier signal is picked up by theantenna I and converted, in a well-known manner, to an intermediatefrequency in the rst detector 2. The intermediate frequency signal istransferred through an'intermediate frequency amplifier 3 to a seconddetector 4. The resulting demodulated signal contains the videocomponent which is amplified in the Video amplier 5, whence it passesdirectly to the grid of a picture tube 6. Suitable deecting signals areproduced in the deflecting circuit I in response to the synchronizingcomponent in the demodulated signal, which component may be derived fromsome point in the video amplifier. The deecting signals may be used toactuate suitable deflecting means,'such as the coil 8, `The .Cil

portion of the' system described thus far and shown schematically is ofconventional form and requires no further description. vThe remainder ofthe system shown in detail and now to bedescribed -constitutes the novelgain control means provided in accordance with the invention.

The signal from which the control is developed may be derived from thelast stage of the intermediate frequency amplier though it will, ofcourse, be understood that this is not a restriction upon the invention.For example, it would also be, possible to use the detected signalobtained from the output of the second detector or from some point laterin the sys-tem, the principal consideration being that the signalutilized should have its D. C. component present, as is the case when itis derived from the intermediate frequency amplifier in the mannershown. If the D. C. component'has been removed it may, of course, .berestored by the use of leveling means in the manner set forth in mycopending application Serial No. 219,643 of which this application is inpart a continuation. This signal may be applied to the grid of the tube9 across a suitable leak resistor I0, the lower end of which may beconnected to a source II of negative bias voltage. The cathode of thetube is maintained at a slightly less negative voltage by connecting itto the center tap of a voltage divider comprising the resistors I2 andI3. The resistor I3, being in the cathode circuit of the tube 9,V issuitably bypassed by the condenser III'. Plate voltage may be suppliedto the tube from a suitable source represented by the battery I5 througha largev plate load resistor I6, which is by-passed by the condenser I1.The tube is so biased that it becomes conducting only when thesynchronizing peaks of the received signal exceed a predeterminedamplitude level, and hence it functions as a detector for the signalpeaks which exceed this amplitude level. Additional smoothing of thevoltage developed across the resistor I6 may be provided by means of theresistor I8 and the condenser I9. The condenser is shunted by a diode 20connected in the sense shown. The filter comprising the resistor I8 andthe condenser I9 is, of course, in thenature of a refinement and may beomitted without essentially modifying the operation of the device. Thecontrol voltage derived from the device maybe supplied to theintermediate frequencyamplifier stages or to some earlier point in thechannel by means of the connection 2| and may be applied to control thegain of the said stages according to any of the conventional methods.

A preliminary consideration of the operation of the system Will showthat no positive voltage can be built up across the condenser I9 byreason of the presence of the diode 20 which acts as a short-circuit toany such voltage. Furthermore,

until a signal is applied to the grid oi the tube 9 which issufficiently large to cause the plate current to increase to a point atwhich the plate goes negative with respect to ground, there will be nonegative volt-age developed across the condenser IQ.' vBut When theplate of the tube 9 becomes negative the diode ceases to conduct Vandythe condenser I9 will be charged through the resistor i8 untilequilibrium has beenY restored. As the plate potential again drops andthe tube 9 becomes less conducting, `the condenser I9 will be dischargedat a much slower rate than that at whichit was charged through theresistors YIt and I8, until another peak of ,signal .occurs-which issuicient to again drive vthe plate negative.v

The' unique advantages inherent in thesystem just described willbe'better understood by refer- 'ence to Fig. 2, and for this purposeitrWill be convenient to disregard the resistor I8 and assume that thediode 20 is connected directly across the plate load resistor I6 and thebattery I5. Fig. 2 shows the plate current-platevoltage characteristicsfor the tube 9 of Fig. 1 functioning as a detector for various peakvalues of modulated intermediate frequency signal. It

should be noted at this point that strictly speaking'the'se" curvescannot be used to describe the operation under these circumstances sinceactually'they represent an ordinary plate family; what is really neededis a detection characteristic.v However, as a iirstapproxirnation madefor the sake of simplicity, they serve to show qualitatively theoperation of the system as shown and would give an accurate picture ofifthe operation in the case where the detected signal'is applied to theAgrid of tube 3. In the absence oi the "diode shunting the Vloadresistor,

and with a smaller resistor in place of IB, the tube might operate onaload line such as that represented by the line ab in Fig. 2. From thisity may be readily seen' that a change in the peak input signal from thevalue 4 to the value 5 will 'cause' theV plate Ycurrent to increase anamount corresponding to thelength of the line dei The value of loadimpedance then being equal to lne/de, vthe slope of the line ab, itfollows that the change in voltage across the lload resistor is equalVto the product toxin/deletek which is designated E1.

f As is well known in the art, and subject to certain practicallimitations, the change in control voltage fora given change lin signalapplied tothe grid may be increased by increasing the load impedance.This appears when we consider Atheload line ac in Fig. 2. YFor a changein peak input signal from value 4 to'value 5 the plate current willchange by the amount corresponding to, the length of the line fg. Theslope of the load line being gh/ fg it follows that thev changefinco'ntr'ol voltage is (measured by gh which is line being Zic/bf itvvill'be` seen that the change` in control voltage is represented by thelength of the line bc which is "designated byrEa. This change in thecontrol voltage will immediately' ibe seen to be much larger than canpossibly be obtained by simply changing the magnitude of the plate loadresistor alone.V Furthermore, the

change in control voltage, which it is possible to effect for a givenchange in the magnitude of the grid signal by applying the method of theinvention, is limited only by practical considerations in the choice ofthe plate load resistor.

Of course, it is now apparent that the greater the change in controlvoltage developed across the resistor I6 for a given change in gridsignal in the device of Fig. l, the more rapid will be the chargingofthe condenser I9, and hence the more quickly the gain vof theintermediate frequency amplier will be changed to compensate forvariations in received signal strength. However, it will be desirable torefer to the diagram oi-Fig. 3 toexplain more clearly the operation ofthe device of Fig. `l when a standard television signal is impressedl onthe grid of the tube 9.

This figure sho-ws a conventionalv plot of plate current versus gridvoltage for a single value of plate voltage, the characteristic beingrepresented by the dotted and solid line kim. 'Ihis is-'modied in thedevice of the invention, by the introduction'of'the diode, to correspondto the heavy line kim. Thus, for all values of grid signal not exceedingthe value n, no plate current will be drawn. When the signal exceedsthis value, plate current increases very rapidly along the form shown at't'.

the line m' to vthe point iwhere it aga-in follows the tubecharacteristic. Ify a television signal whose envelope is represented byp be applied to the grid of the tube, the synchronizing peaks q andsmall portions of the signal belowthe blanking level T will tend toexceed the level designated n-s corresponding to cut-ofi in the tube.When this condition obtains, the impulses of plate current which areAproduced will ybe of 'Ihe magnitude of the control signal as comparedwith the magnitude of the input signal will be immediately apparent.

It will be well, at this point, toV mention the delay in Ythe operationof the system which obtains by the use'of the diode in the output of theautomatic gain Icontrol tube. Since no control voltage is developedacross the diode until the'plate oi the gainY control tube goesnegative, the point at which the control begins to operate may bedenitely iixed by suitably biasing the diode, or by `varying the othervoltages and circuit elements. VIn Fig. 1 the cathode of the diode isgrounded, but it will be clear that a battery or other convenientvoltage source may be insertedto maintain the diode lcathode at anydesired potential whereby the threshold of control is iixed. Theexceptionally large change in control voltage which obtains` when thethreshold is reached may be regarded as a result of achieving the delayaccording to the method of the invention. By the cooperation of thesetwo features of delay and exceptionally large control, it is possibletoobtain what is referred toV in the art as avery nat gain control. Y

A further important detail relating to the mode of operation of thesystem will also be apparent from the/diagram of Fig. 3. The amount ofcontrol is dependent upon the areas of the plate current pulses whichare designated tand by far the greater part of this ai'ea corresponds tothe portion of the input signal below theV blanking level r as com-paredwith that which corresponds to the synchronizing component of the inputsignal. The portion of the signal below the blanking level used to formthe control signal must necessarily `be keptvsiall in` order that thebackground component be not removed lfrom .the

video signal. This is fully explained in applicants above-mentionedcopending application. In order to make the best possible use of thissmall portion below the blanking level, it is desirable that the averageamounts by which the blanking level r exceeds the cut-olf level nsshould correspond to the change in grid voltage along the characteristiccorresponding to the steep portion of the characteristic m' in Fig, 3.This permits of the greatest degree of control with the utilization of aminimum of the blanking signal below the level, the resultant inclusionof the least possible amount of video component, and hence the leasttendency to destroy the D. C. or average brightness component containedin the video signal. 'I'he amount of signal utilized is dependentfundamentally upon the size of the plate load resistor i6 of Fig. 1 andthe bias, if any, applied to the diode 20. This will be clear byreference to Fig. 3 where it will be seen that the desired operationobtains when the average value of the blanking level corresponds to avalue of grid voltage on the part of the characteristic designated n--ior slightly greater (i. e. less negative) but preferably not less.

It will be understood that the present disclosure is illustrative onlyand is not intended to impose any limitation upon the invention, thescope of which is defined by the appended claims.

I claim:

1. In a television system of the type employing a composite signal`having video and synchronizing components established on opposite sidesof a predetermined blanking level, a signal channel, means for derivinga signal from said channel, a gain control tube operable by said derivedsignal, means for causing the eifective plate current of said tube todecrease rapidly and abruptly from a substantial value to cut-oli, meansfor adjusting the operation of said tube in predetermined relation toits cut-off point such that the blanking level normally exceeds thecut-oir point of said tube but not sufciently to permit the tube outputto include an appreciable portion of the video component, whereby thetube output is substantially independent of variations in the videocomponent, and means for applying the output signal from said tube tosaid channel to control the gain thereof.

2. In a television system of the type employing a composite signalhaving video and synchronizing components established on opposite sidesof a predetermined blanking level, a signal channel, means for derivinga signal from said channel, a gain control tube operable by said derivedsignal, means for causing the effective plate current of said tube todecrease rapidly and abruptly from a substantial value to cut-oir, meansfor adjusting the operation of said tube in predetermined relation toits cut-off point such that the average value of the blanking levelexceeds the cut-ofi point of said tube but not sutiiciently to permitthe tube output to include an appreciable portion of the videocomponent, whereby the tube output is substantially independent ofvariations in the video component, and means for applying the outputsignal from said tube to said channel to control the gain thereof.

3. In a television system of the type employing a composite signalhaving video and synchronizing components established on opposite sidesof a predetermined blanking level, said synchronizing componentcomprising time-spaced pulse signals occurring during intervals ofduration greater than said pulse signals and in which said video signalcomponent is established at said blanking level, a signal channel, meansfor deriving a signal from said channel, a gain control tube operable bysaid derived signal, means for causing the eiective plate current ofsaid tube to decrease rapidly and abruptly from a substantial value tocut-oil, means for adjusting the operation of said tube in predeterminedrelation to its cut-01T point such that the blanking level normallyexceeds the cut-ofi point of said tube but not suliiciently to permitthe tube output to include an appreciable portion of the videocomponent, whereby the tube output is substantially independent ofvariations in the video component, and means for applying the outputsignal from said tube to said channel to control the gain thereof.

4. In a television system of the type employing a composite signalhaving video and synchronizing components established on opposite sidesof a predetermined blanking level, a signal channel, means for derivinga signal from said channel, a gain control tube having at least ananode, a cathode and a control grid, means for supplying said derivedsignal to the control grid of said tube, a load impedance for said tubecharacterized in that it decreases suddenly in magnitude when the platecurrent in the tube falls below a predetermined value, thereby causingthe plate current to decrease rapidly for a small change in grid voltagewithin a certain range, means for so operating said tube that theaverage value of the blanking level of the applied composite signalexceeds the grid voltage corresponding to cut-oli of the tube but notsuiciently to permit the tube output to include an appreciable portionof the video component, whereby the tube output is substantiallyindependent of variations in the video component, and means for applyingthe output signal from said tube to said channel to control the gainthereof.

5. In a television system employing a composite signal having video andsynchronizing components established on opposite sides of apredetermined blanking level, a signal channel, means for derivinga-signal from said channel, a gain control tube having at least ananode, a

y cathode, and a control grid, means for supplying said derived signalto the control grid of said tube, a load impedance for said tube, aconnection including a diode between a point on said load impedance anda point of predetermined potential, for effectively causing said loadimpedance to decrease suddenly in magnitude when the plate current inthe tube falls below a predetermined value, thereby causing the platecurrent to decrease rapidly for a small change in grid voltage within acertain range, means for so operating said tube that the average valueof the blanking level of the applied composite signal exceeds the gridvoltage corresponding to cut-off of the tube, but not suiiiciently topermit the tube output to include an appreciable portion of the videocomponent, whereby the tube output is substantially independent ofvariations in the video component, and means for applying the outputsignal from said tube to said channel to control the gain thereof.

ALEXANDER R. APPLEGARTH, JR.

